Interaction of Nitrogen and Potassium Nutrition on Growth and Root Physiology of Alfalfa

R. Li, B.C. Joern and J.J. Volenec
Dept. of Agronomy, Purdue University, West Lafayette, IN 47907-1150

Potassium (K) deficiency reduces alfalfa (Medicago sativa L.) shoot growth as well as root protein and carbohydrate concentrations. Nitrogen (N) fertilizer may replace N normally acquired via N₂-fixation, a process that is severely reduced by K deficiency. Our objective was to determine if N fertilization alters shoot growth, and root carbohydrate and protein accumulation in alfalfa plants receiving varied levels of K. 'Resistar' alfalfa plants were defoliated and transplanted into quartz sand where they were provided Hoagland's solutions containing 10 mM N as NO₃^- or NH₄⁺ in 0, 3, and 6 mM K for 90 days. Plants were defoliated at 30-day intervals. Plants were sampled immediately after the third defoliation, and at 7-day intervals thereafter during 28 days of regrowth. At sampling, plants were separated to shoots, roots, and crowns. Roots were washed free of sand, lyophilized, and milled to pass a 1-mm screen. Root tissues were analyzed for total N, K, sugar, starch, and buffer-soluble protein.

Root K concentrations increased as K levels of the nutrient solution increased. Root K concentrations were similar for all N treatments when plants were supplied 0 and 3 mM K, whereas, root K concentration was reduced in NH₄⁺-treated plants receiving 6 mM K. Root N concentrations were low at 0 mM K, and increased with K nutrition. Addition of N increased root N concentrations only in plants receiving 0 mM K. The NH₄⁺-treated plants accumulated slightly higher root N levels at 3 and 6 mM K when compared to NO₃^--treated and control plants. Survival of NH₄⁺-treated plants was reduced, but survival improved as solution K levels increased. Shoots per plant increased as solution K concentration was raised from 0 to 3 mM. For control and NO₃^--treated plants, mass per shoot increased markedly as K was raised from 0 to 3 mM, whereas 6 mM K was required to increase mass per shoot of NH₄⁺-treated plants. Root mass per plant was reduced by NH₄⁺ application. Root starch concentrations increased significantly as solution K concentration increased from 0 to 6 mM, and were severely reduced in NH₄⁺-treated plants. Root sugar concentrations were unaffected by K nutrition. Root protein concentrations were not affected by N application, but increased as solution K concentration increased from 0 to 3 mM.

Application of N as NO₃^- had little impact on growth or taproot physiology of alfalfa irrespective of K nutrition. Alfalfa persistence, shoot growth, and taproot starch were reduced by NH₄⁺ applications, especially in plants receiving 0 mM K.

1995 Central Alfalfa Improvement Conference Proceedings